LED illumination using a light emitting diode (LED) has become popular now because it offers power saving and long life.
An LED element has low electrostatic resistance. Hence, when the LED element is simply and directly mounted on a motherboard, it is highly likely to be broken by direct influence of static electricity resulting from the motherboard or static electricity applied from the outside via the motherboard. To avoid this, it is conceivable to connect an electrostatic protection element to the LED element. However, when the LED element is directly mounted on the motherboard, the electrostatic protection element also needs to be mounted near the LED element. This increases the total mounting area, and miniaturization is not achieved.
Accordingly, for example, Japanese Unexamined Patent Application Publication No. 11-251644 and Japanese Unexamined Patent Application Publication No. 2008-270327 describe a structure in which an LED element is mounted on a motherboard with a mounting substrate, including a base material of silicon single crystal or a ceramic material, being disposed therebetween. There is a method for providing the mounting substrate with the function of the above-described electrostatic protection element.
Such a mounting substrate is shaped like a flat plate. An LED element is mounted on a front side of the mounting substrate, and a back side of the mounting substrate is mounted on a motherboard. For this reason, component mounting conductors are provided on a front surface of the base material of the mounting substrate, and external connection conductors are provided on a back surface of the base material. To ensure continuity between the component mounting conductors and the external connection conductors, connection conductors are provided to penetrate the base material.
For example, FIGS. 9(A), 9(B), and 9(C) show a front view, a sectional side view, and a back view of an example of a mounting substrate of the related art.
A mounting substrate 10P1 includes a base material 20 shaped like a flat plate. On a front surface of the base material 20, component mounting conductors 31 and 32 are provided. On a back surface of the base material 20, external connection conductors 41 and 42 are provided. The component mounting conductor 31 and the external connection conductor 41 are provided to be superposed on each other when the base material 20 is viewed in plan. The component mounting conductor 32 and the external connection conductor 42 are provided to be superposed on each other when the base material 20 is viewed in plan. The component mounting conductor 31 and the external connection conductor 41 are coupled by a conductive via 51P1 penetrating the base material 20 in the thickness direction. The component mounting conductor 32 and the external connection conductor 42 are coupled by a conductive via 52P1 penetrating the base material 20 in the thickness direction.
The conductive vias 51P1 and 52P1 are filled with a conductive material. When conductive vias 51P1 and 52P1 are used, they need to have a small diameter to prevent the substrate from being broken by thermal stress of the conductive material.
However, the difficulty in filling the conductive material increases as the diameter decreases. Hence, it takes much time to fill the conductive material.
For this reason, for example, structures using through holes illustrated in FIGS. 10(A) through 10(C) and 11(A) through 11(C) have been hitherto used. FIGS. 10(A), 10(B), and 10(C) and FIGS. 11(A), 11(B), and 11(C) each show a front view, a sectional side view, and a back view of a different example of a mounting substrate of the related art.
In a mounting substrate 10P2 illustrated in FIGS. 10(A), 10(B), and 10(C), the conductive vias 51P1 and 52P1 of the mounting substrate 10P1 are replaced by conductive through holes 51P2 and 52P2. Other structures are the same.
The conductive through holes 51P2 and 52P2 include holes 511P2 and 521P2 having a certain diameter, respectively. The hole 511P2 penetrates a component mounting conductor 31, a base material 20, and an external connection conductor 41. The hole 521P2 penetrates a component mounting conductor 32, the base material 20, and an external connection conductor 42. A conductor film 512P2 is provided on an inner wall surface of the hole 511P2. A conductor film 522P2 is provided on an inner wall surface of the hole 521P2.
In a mounting substrate 10P3 illustrated in FIGS. 11(A), 11(B), and 11(C), structures of conductive holes 51P3 and 52P3 are different from those of the conductive through holes 51P2 and 52P2 of the mounting substrate 10P2. Other structures are the same.
The conductive holes 51P3 and 52P3 include holes 511P3 and 521P3 having a certain diameter, respectively. The hole 511P3 penetrates an external connection conductor 41 and a base material 20, and reaches a surface of a component mounting conductor 31 on the base material 20. The hole 521P3 penetrates an external connection conductor 42 and the base material 20, and reaches a surface of a component mounting conductor 32 on the base material 20.
A conductor film 512P3 is provided on an inner wall surface of the hole 511P3. The conductor film 512P3 is provided not only on the inner wall surface of the hole 511P3 in the base material 20, but also on the inner wall surface of the hole 511P3 at the component mounting conductor 31. A conductor film 522P3 is provided on an inner wall surface of the hole 521P3. The conductor film 522P3 is provided not only on the inner wall surface of the hole 521P3 in the base material 20, but also on the inner wall surface of the hole 521P3 at the component mounting conductor 32.